Atrioventricular septal defects (AVSDs) are congenital heart malformations found in approximately 5% of all individuals suffering from congenital heart disease (CHD). They are particularly common in individuals with Down Syndrome (DS) and in patients with heterotaxy syndrome. Two AVSD subtypes can be distinguished; incomplete AVSDs, where shunting is restricted to the atrial level via an ostium primum atrial septal defect (pASD) and complete AVSDs with shunting at atrial as well as ventricular level (via an inlet type VSD). While it was believed for many years that abnormal development of atrioventricular cushions was the only mechanism involved in the pathogenesis of these defects, more recent studies have revealed that perturbation of tissues derived from the posterior Second Heart Field (pSHF) located at the venous pole of the heart, including the primary atrial septum and the Dorsal Mesenchymal Protrusion (DMP), play a critical role in the pathogenesis of AVSDs as well. In this application we propose to investigate the synergistic relationship between three signaling pathways that are involved in the regulation of pSHF/DMP development and in the formation of the AV septal complex, i.e. the Hedgehog (Hh) signaling pathway, the Wnt(2)/-catenin pathway, and the BMP signaling pathway. In addition, we will study the role of primary cilia in the SHF, as abnormalities in the function and/or structure of ciia are associated with the pathogenesis of AVSDs. The synergy between the two aims is found in the fact that the molecular mechanisms implicated in pathogenesis of AVSDs investigated in aim 1 are also involved in the assembly and/or function of primary cilia. Finally, using a number of conditional knock out models we aim at obtaining insights into the differences in the pathogenesis of incomplete vs complete AVSDs, testing the hypothesis that the pASD found in all forms of AVSDs exclusively results from perturbation of DMP development, while additional abnormal developmental events in the AV cushions and ventricular myocardium are responsible for the defects present at ventricular level and in the AV valves.